2098 



A TEXTBOOK OF THEORETICAL BOTANY 



Fig. 2034. — Platanthera chlorantha. A, Flower. B, Column showing the two triangular 

 retinacula which attach themselves to the eyes of the insect. C, PoUinium with caudicle 

 and retinaculum. See in text. (A after Soiverby. B and C after Kniith.) 



The genus Orchis itself has a relatively large number of British species. 

 Compared, however, with many of the tropical genera it is small, for it 

 contains only about seventy species. They are distributed throughout the 

 northern hemisphere in temperate regions, a dozen of them being found in 

 Britain (see above). The species of this genus are perennials and form one 

 tuber each year. The flowers are produced in dense spikes and the leaves 

 of some species are spotted with brown and purple. 



As is usual in the family, the flowers are adapted to insect visits and have 

 received considerable attention in this respect. Darwin in his book on 

 " The Fertilization of Orchids " devotes considerable space to the genus, 

 and students are referred to this book for a more detailed account of the 

 floral structure than can be given here. For our present purpose we may cite 

 one example. 



In Orchis mascula (Fig. 2035) the flowers are somewhat larger than those 

 of other species and the labellum provides a good alighting place for an 

 insect. The pouch-shaped rostellum projects above the entrance to the 

 nectary so that an insect probing for nectar must inevitably touch it. In so 

 doing it ruptures the membrane of the rostellum and depresses it. This 

 uncovers the retinacula so that one or both the pollinia become implanted 

 on its head. It should be noted (as can be easily ascertained by inserting a 

 pencil in a flower) that the mucilage hardens very rapidly, so that almost as 

 soon as the pollinia have been brought out of the flower they cannot easily be 



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